With modern microfabrication-based manufacturing techniques, mass production of matrix-addressed, electronic flat-panel displays (FPDs) has become routine. Alternative forms of such displays are currently based on a number of different technologies, including liquid crystal (active-matrix, or AMLCD, and passive-matrix, or STN), plasma, field emission and thin CRT, as well as electroluminescence (EL). A flat, rectangular viewing area is generally provided, as are two sets of orthogonal electrical lines, one for data, and the other for control.
Few attempts have been made, heretofore, to fabricate large-sized FPDs. One approach has been to construct the generally large viewing area by using an array of smaller, discrete FPDs, or tiles. Some of the construction techniques for these large displays are discussed hereinafter.
The color image on an electronic display is produced by an aggregate of individual picture elements, called pixels, that comprise the viewing area of the display. Each pixel has additional internal structure, usually three, discrete, sub-pixels that generate the primary colors, with the usual choices being red, green and blue. It is convenient to assume that the color coordinates of each primary-color type of sub-pixel (e.g., green) do not vary so much as to be objectionable to the average viewer over the area of a single tile (intra-tile) or between tiles (inter-tile). Where this condition is not met, methods for correcting tiled FPDs are described in co-pending patent applications bearing Ser. Nos. 618,046 (filed Mar. 25, 1996); 628,308 (filed Apr. 5, 1996); 636,604 (filed Apr. 23, 1996), and 649,240 (filed May 14, 1996), all of which are hereby incorporated by reference.
Each sub-pixel is further characterized by a monotonic input-signal versus luminance relationship. An electrical characteristic of the input signal (such as DC level, rms or peak-to-peak AC level, frequency, pulse width, etc.) sent to each sub-pixel is varied to produce a desired amount of light corresponding to the sub-pixel's color (e.g., red luminance) in order to create that part of the image represented by that primary color at that pixel. A typical, tiled FPD might have 640 columns of pixels by 480 rows of pixels, or, 307,200 red, green and blue sub-pixels.
The ultimate objective in producing a high-quality, color representation of the target image on a display is is the same, or within the color and brightness discrimination limits, for all sub-pixels of the same color across the entire display. This is not normally achieved in mass-produced displays for consumer products such as television sets and computer monitors.
Viewers of television sets and computer monitors (in both CRT and liquid-crystal displays) usually accept gradual luminance variations on the order of 20 percent between one region of the display and a remote region (center-to-corner, e.g.). This is acceptable to the average viewer, because the human eye does not perceive small gradual changes in luminance or chromaticity as objectionable. Display devices and systems that meet this condition are said to obey a "low-gradient" rule.
Large changes in luminance or chromaticity between one region of the display (having any large number of contiguous pixels) and another region where the change occurs across a narrow, distinct boundary region (e.g., one or a few pixels wide) are not acceptable to the average viewer. Under ideal viewing conditions, abrupt changes of one percent or less can be objectionable; under less-than-ideal conditions, the objectionable threshold could be much higher, perhaps as large as ten percent. Display devices and display systems that have this objectionable condition are said to have "high gradient". The criteria of both human visual perception and display quality (especially with respect to luminance and chromaticity gradients) for tiled FPDs are described in more detail in the aforementioned co-pending patent applications.
Because of the nature of manufacturing processes, in conjunction with market forces, consumer display systems usually meet the low-gradient rule. However, it is quite possible, and even likely, that individual FPD devices, panels or tiles meeting the low-gradient rule will, when assembled to make a larger, tiled display, exhibit the high-gradient condition. The methods, designs, processes, manufacturing and set-up procedures to produce such a display are known as "tiling". The high-gradient condition is most likely to occur across the boundary, or, the seams between the tiles. An even more egregious condition may occur at corners where four tiles meet. At such a so-called saddle point, a positive luminance gradient with respect to the corner point may exist in one direction therethrough, and a negative luminance gradient with respect to said corner point along another line distinct from the first one.
Since a tiled FPD made from panels that meet the low-gradient rule is likely to exhibit the unacceptable, high-gradient condition, a new and unique set of problems must be solved to make tiled displays visually acceptable. Novel inventions are required for tiling, if the resultant display is to meet the low-gradient rule.
Color matching is any technique used to produce a tiled FPD that meets the low-gradient rule, via individual tiles or other display system components or subsystems (e.g., a backlight), which might contribute to a resultant, high-gradient condition without the application of such methods. Color matching is a necessary but not sufficient condition for producing a tiled display which has the equivalent visual appearance to the viewer of a monolithic display without any seams.
Color-matching methods may be classified into two broad categories, transformation of data and display set-up. In the aforementioned co-pending patent application, bearing Ser. No. 618,046, color-matching methods achieved by display set-up techniques are described. These include spatially varying the neutral density filter and using tile- and sub-tile-based set-up parameters (such as reference and cutoff voltages, for example). These techniques do not change or transform the input video data; rather, they change the display characteristics (such as gain, offset and optical density).
The aforementioned co-pending patent applications describe, variously, color-matching methods that are based on transforming data derived from smoothing algorithms, splines, coefficient tables, etc., or that are based on transforming data via photometric or calorimetric relations. These methods have in common at least some real-time computation to transform data, the storage of coefficients necessary to perform the data transformation, and algorithms and hardware to implement the methods.